When wastewater infiltrates into the soil, groundwater may be contaminated. If the distance from source of pollution to point of groundwater abstraction is small, there is a real chance of abstracting pathogenic microorganisms. This book studies the transport of Escherichia coli in aquifers under saturated conditions. The author uses the well known colloid filtration theory as a starting point. He then includes physical sieving, geochemical heterogeneity, variable deposition rate coefficients, and preferential flow mechanisms in the theory. The case of Sana'a evaluates the importance of bacteria transport processes in a field situation.